ISO C++ forbids zero-size array 'iso_packet_desc'

[dsd]

from: ​https://sourceforge.net/tracker/?func=detail&aid=2740374&group_id=1674&atid=101674

libusb.h causes errors in C++ projects that use GCC's -pedantic switch:

error: ISO C++ forbids zero-size array 'iso_packet_desc'

One solution is to make the array size 1 and account for that change in all
places that use sizeof(struct libusb_transfer), e.g.:

int alloc_size = sizeof(struct usbi_transfer)
+ sizeof(struct libusb_transfer)
+ (sizeof(struct libusb_iso_packet_descriptor) * (iso_packets-1))
+ os_alloc_size;

OK. Then I have a possible solution which I don't completely dislike:

use an #ifdef to make the array size 1 when using a C++ compiler, but
leave the other situations as-is.

Outside of the header file, no libusb changes are needed since libusb will
always be compiled with a C compiler.

What do you think?

[xiaofan]

Just wondering how to reproduce this one and wonder if this is valid or not.

mcuee@ubuntu:~/Desktop/build/libusb1/test$ g++ -pedantic test.cpp -o test -lusb-1.0
mcuee@ubuntu:~/Desktop/build/libusb1/test$ cat test.cpp
#include <libusb-1.0/libusb.h>
#include <iostream>
using namespace std;

int main()
{

int i=0;
libusb_context c = NULL;
i = libusb_init(c);
cout<<"

using libusb.h
";

return 0;

}

mcuee@ubuntu:~/Desktop/build/libusb1/test$ g++ -pedantic test.cpp -o test -lusb-1.0
mcuee@ubuntu:~/Desktop/build/libusb1/test$ gcc -v
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.4.3-4ubuntu5' --with-bugurl=​file:///usr/share/doc/gcc-4.4/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 --program-suffix=-4.4 --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-plugin --enable-objc-gc --enable-targets=all --disable-werror --with-arch-32=i486 --with-tune=generic --enable-checking=release --build=i486-linux-gnu --host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5)

[hansdegoede]

Using 0 sized arrays at the end of a struct for a variable sized array is quite a normal thing to-do now a days in plain C.

Since this is only a warning and only happens with -pedantic, and likely only with older gcc versions I suggest we just close this one. Also note that the proposed fix would be an ABI change which is a no no.

[hjelmn]

I agree with Hans. I am setting a timeout of a week then I will close this ticket as wontfix. If there is any interest in fixing this (though I don't think this is a defect) let me know.

[LudovicRousseau]

Two possible fix:

​https://github.com/mmuman/contiki/commit/1d900be802e20ceda323bc44b753a449853f4278

​https://github.com/LudovicRousseau/libusbx/commit/53134e90f2f8fc516cc9794d79e754703da9e894

I prefer the first implementation.

[hjelmn]

Ugh, neither option is very clean. I agree the first implementation is better. I will adapt it for libusb and push it to a branch on my github tree.

Since this doesn't break API compatibility I will go ahead and mark this for 1.0.16.

[hjelmn]

Hmm, we already handle this in the same way as ​https://github.com/LudovicRousseau/libusbx/commit/53134e90f2f8fc516cc9794d79e754703da9e894 in libusb.h so there is no reason to not do it the same way for libusbi.h. I will just pull the patch from your tree.